In the paper industry, drainage is implemented using suction pipes, suction boxes, suction rollers or equivalent provided with longitudinal gaps or holes against a wire screen, felt or equivalent which carries the wet web of paper. Each suction pipe is so placed that the wire, felt or equivalent passes over the gap or holes and drying is effected as the water is hurled by a centrifugal force into the roller trough. The water is gathered in the suction pipe and passed to a suitable collecting place. An apparatus of this type is presented in Finnish patent publication 77706 (Corres. to U.S. Pat. No. 4,329,201). In current systems, the vacuum is adjusted on the basis of pressure, by using various valve elements or by varying the rotational speed of a pump unit to maintain a desired pressure. Of the processes in a paper machine, the vacuum system is responsible for the highest power consumption. It also has an effect on the driving output and steam consumption of the press and wire section. In the solutions currently used, the adjustment of the vacuum capacity affects the negative pressure, but it has not been possible to reliably determine the effect of the adjustment on the drainage of the paper machine. An excessively high vacuum level is not beneficial to drainage. Instead, it increases the pumping costs and the costs resulting from the wear of fabrics and consumption of energy caused by increased friction.
The object of the present invention is to achieve a completely new solution that allows the drawbacks of known technology to be avoided.